Ever since the oil embargo and resulting energy crisis of the late 1970's, America has been striving to develop the highest degree of energy independence possible by stressing the need for conservation of our natural resources. Greenpeace has been instrumental in focusing attention on the threat, real or imagined, of the "green house effect", which has also spurred efforts to reduce energy consumption. Much quantitative data has been generated documenting the tremendous potential energy savings with induction motor energy conservation schemes described in several previous patents.
Interest in this technological area was initially spurred on by NASA's Tech Brief entitled "Improved Power-Factor Controller" and resulting U.S. Pat. No. 4,266,177 issued to Nola. The research giving rise to the paper and patent documented power savings for a broad line of induction motors from 1/3 to 5 horse power of both single and three phase varieties. Subsequent research efforts have demonstrated that power savings schemes different from those developed by NASA can produce similar levels of savings.
The power factor controller developed by NASA is, unfortunately, only a power factor controller and does not maximize power savings. While there is a strong correlation between power factor control and energy savings, significant potential savings may be unrealized in many instances since minimization of power will not occur in a control scheme that sets a specific operation phase angle rather than an optimal varying point. This latter more beneficial control is available with a load-feedback-driven servo controller of the type described herein incorporating a unique feedback scheme. Other problems may arise with a set point phase-angle controller in that all operating conditions may not have been considered when setting an operating point, and insufficient voltage may be provided to the motor at critical times which can cause system operational problems, excessive energy consumption or even product failure. Compensation for such problematic situations requires detuning of such phase-angle controllers which always result in reduced operational improvements; so, predicted, published results are not realistically obtainable for NASA's invention when utilizing existing prior art systems. U.S. Pat. No. 4,413,217 points this out when it states, "However, it is known that such full load phase lag varies for each motor and should be set on the controller in accordance with the specific motor being utilized."
Other U.S. Pat. Nos. 4,806,838 issued to Weber and 4,413,217 issued to Green et al., each teach the use of power factor controllers and use multiple feedback signals to accomplish their power savings improvements. The circuits described in these patents are quite complicated, require many components, and only marginally improve, rather than maximize, power factors characteristics and some energy savings. Another invention discussed in U.S. Pat. No. 4,477,761 presents a circuit of larger complexity using multiple feedback signals, and steps through regulating minimum power consumption in a bang-bang servo. This could result in nuisance, audible noise variation when going from speed to speed, as with other stepped controllers, including the power factor controllers. Another disadvantage of such stepping controllers is that the time lag between steps can actually cause the loss of some operating efficiency improvement because time-varying parameters will have changed prior to the implementation of the next step. U.S. Pat. No. 4,333,046 refers to a 3-phase system using only two power factor controllers to effectively reduce a portion of the equipment cost of a 3-phase system using three controllers by one-third. Such a system would benefit from using the embodiment of this invention by providing optimum energy savings, as well as from the other benefits of the present invention.
Stepping controllers of the type used in the prior art can introduce mechanical stress in the form of torque pulsations resulting from the sudden change of applied voltage during stepping. This mechanical stress results in wiping film lubrication from bearing and shaft surfaces. This film wiping reduces bearing life and consequently the life of the motor and load it is driving.
In the present invention, the disadvantages of a set point controller are overcome by a true load-driven feedback servo controller system , thereby providing optimal tuning, maximum savings, and improvement throughout the operating range from no load all the way to full load. The use of a true load-driven feedback servo controller delivers operational improvement in energy consumption reduction even at full loading because electrical power utilities provide for .+-.10% voltage variation in supplied user voltage, which implies a system will work all the way to -10% nominal voltage. It can, therefore, be inferred that there is an excess of 10% operation voltage applied to a motor at nominal operating voltage, and it is this potential 10% reduction in operating voltage that may be targeted by a load-driven feedback servo controller, thereby allowing for energy consumption reduction all the way to full loading of a motor. This true load-driven feedback servo controller approach can be applied to any type of energy conservation system, including series triac voltage reduction systems like this invention discusses, variable-voltage transformer drive systems, and systems detailed in the prior art patents described herein.
Specifically, this invention has been applied to a variety of motors in the same horse power range discussed in the NASA Tech Brief and significant savings have been demonstrated throughout the range from no load to full load, on motors of all sizes. The single most important feature of this invention is that the regulation is completely automatic, and that maximum savings and satisfactory operation of motors is achieved over the complete motor operating voltage and load ranges. This is because the automatic minimum power consumption circuit forces continuous improvement even at full load. No sacrifice is required as is the case with NASA's power factor controller and others and there is always some improvement, a minimum of 2% having been demonstrated by empirical tests.
This power saving circuit is intended for passive, unidirectional power transmission, and as such, must supply positive power every half cycle of applied power. A common error in most power factor controllers and power savers is to supply two or four quadrant (or regenerative circuits) to fire the triac. This may be intentional, but is believed to usually be an unintentional result of misunderstanding firing and ramp generating circuits. A load on a motor is generally constant, which reflects a minimum applied voltage required to a motor. Phase controlling any of these controllers from 0.degree. to 180.degree. will negate this requirement and will, in fact, cause the load to overhaul the motor and cause the motor to slow down, vibrate or even stall. This is one of the biggest problems with all of the prior art controllers, and is eliminated by reducing the maximum phase shift in the firing circuit (or firing delay) to a value that can be calculated from system operating characteristics.
For this power saver, nominal line variations of .+-.10% produces the following result: If a motor is running with +10% applied nominal voltage, and the motor-load combination will run satisfactorily at -10% applied nominal voltage, the maximum reduction available would be -20% applied voltage. To accommodate transients, stepped load changes, etc., control systems engineering standards dictate twice that value for proper feedback controller operation. Therefore, 40% reduction in applied voltage is all that is required for proper controller operation. Providing a nominal increase in phase delay would increase 40% slightly, but generally not above 50% (i.e., provide a safety factor). This minimizes the firing phase delay from 0.degree. to 90.degree. delay, maximum. Because of this, an accommodation must be made in the ramp generator, timing summing point to force firing of the triac at no later than 90.degree. phase delay. This will cause smooth, continuous, maximum power savings 100% of the time with no controller or motor malfunction or failures, which are common with other phase angle or power savings controllers.
Eventually, though, every man-made mechanical device will wear and start to fail, including any motor driven by the controller of this invention. As a device starts to wear, fail, or experience other problems, such as worn bearings, the feedback of this invention starts to pulse or vary outside predetermined limits and begins to fully fire the triac, or what ever control means is utilized, and effectively no voltage reduction occurs, for short periods of time initially, or continuously, as more wear develops. This inherently produces full voltage firing which is similar to that which occurs when a transient or step change in load occurs in this other systems. The integrated error amp in this controller will saturate and cause the firing circuit to fully fire, to ride through the momentary overload by supplying full line voltage for a few seconds. If the overload is maintained because of wear or other problems, the error amp becomes continuously saturated and fully fires the controller indefinitely.
The present invention is equipped with a light or some signaling device which can be driven when predetermined voltage levels occur above and below a preset value across a voltage limiting device driven by a servo controller. Most preferably, a dual color light-emitting diode of red and green is used, red signaling no savings or a problem, and green indicating savings and a well operating servo controller and motor. Typical problems that can be indicated by such a red/green LED include the full firing operation referenced above or, in the case of a refrigeration/air conditioner unit where low freon will cause a compressor to surge or pulsate. Problems such as these will result in a voltage value below the threshold voltage thus activating the red color of the signaling LED. In actuality, any motor-driven load system problem that causes the feedback to become saturated or unstable will cause the firing circuit of a servo controller built to the criteria of this invention to signal red in a red/green LED signal indicator.
It is among the objects of this invention to provide a simpler and more efficient system, that is, one requiring fewer components, minimizes energy consumption, reduces mechanical stresses, and prolongs the life of the motor and load it drives. Also among the objects is the desire to provide a signaling means to indicate energy savings and potential fault conditions in either the motor or the load driven by that motor. Further, it is among the objects of this invention to describe a compressor cycling system to decrease operating costs of refrigeration/air conditioning equipment by up to 50%.
In accordance with this invention, a minimal energy consumption power controller is constructed wherein the negative derivative of either current or power with respect to voltage applied to a motor is measured by a feedback current (or power) sensor; amplified; filtered; converted to a representative voltage; and compared to a reference bias voltage of opposite polarity, proportional to the value of the negative derivative by an integrating error amplifier; and synchronously compared to a zero voltage initiated, ramp-generated signal to produce a train of firing pulses to a triac (or other voltage regulating device). The firing pulses to the triac are therefore synchronously timed to deliver a delayed turn-on command in a servo controller and provide a smooth, reduced level of voltage to a motor. This has the effect of minimizing the power input to said motor. An indicator is also provided to indicate this power reduction and also warn of potential faults with either the motor or load to which the motor is connected.
Additional features, advantages and characteristics of the present invention will become apparent after a reading of the following detailed description.